The present invention relates to a black magnetic toner and black magnetic.composite particles for the black magnetic toner, and more particularly, to black magnetic composite particles for high-resistant black magnetic toner, which are not only more excellent in fluidity and blackness but also show an excellent dispersibility in a binder resin due to a less amount of carbon black desorbed or fallen-off from the surface of each particle; a process for producing the black magnetic composite particles; and a high-resistant black magnetic toner using the black magnetic composite particles which is more excellent in fluidity and blackness.
As one of conventional electrostatic latent image-developing methods, there has been widely known and generally adopted a so-called one component system development method of using as a developer, a magnetic toner comprising composite particles prepared by mixing and dispersing magnetic particles such as magnetite particles in a resin, without using a carrier.
The conventional development methods of using one-component magnetic toner have been classified into CPC development methods of using a low-resistance magnetic toner, and PPC development methods of using a high-resistant magnetic toner.
In the CPC methods, the low-resistance magnetic toner used therefor has an electric conductivity, and is charged by the electrostatic induction due to electric charge of the latent images. However, since the charge induced on the magnetic toner is lost while the magnetic toner is transported from a developing zone to a transfer zone, the low-resistance magnetic toner is unsuitable for the PPC development method of using an electrostatic transfer method. In order to solve this problem, there have been developed the insulated or high-resistant magnetic toners having a volume resistivity as high as not less than 1xc3x971013 xcexa9xc2x7cm.
As to the insulated or high-resistant magnetic toner, it is known that the developing characteristics thereof are affected by magnetic particles exposed to the surface of the magnetic toner, or the like.
Recently, with the high image quality such as high image density or high tone gradation, or with the high copying speed of duplicating machines, it has been strongly demanded to further enhance characteristics of the insulted or high-resistant magnetic toners as a developer, especially a fluidity thereof.
With respect to such demands, in Japanese Patent Application Laid-Open (KOKAI) No. 53-94932(1978), there has been described xe2x80x9cthese high-resistant magnetic toners are deteriorated in fluidity due to the high electric resistance, so that there arises such a problem that non-uniformity of developed images tend to be caused. Namely, although the high-resistant magnetic toners for PPC development method can maintain necessary charges required for image transfer, the magnetic toners are frictionally charged even when they are present in other steps than the transfer step, where the magnetic toners are not required to be charged, e.g., in a toner bottle or on the surface of a magnetic roll, or also slightly charged by mechano-electrets during the production process of these magnetic toners. Therefore, the magnetic toners tend to be electrostatically agglomerated, resulting in deterioration of fluidity thereofxe2x80x9d, and xe2x80x9cIt is an another object of the present invention to provide a high-resistant magnetic toner for PPC development method which is improved in fluidity, can be prevented from causing non-uniformity of developed images, and has an excellent image definition and tone gradation, thereby obtaining high-quality copies by indirect copying methodsxe2x80x9d.
In recent years, with the reduction in particle size of the insulated or high-resistant magnetic toners, it has been increasingly required to enhance the fluidity thereof.
With respect to such a fact, in xe2x80x9cComprehensive Data Collection for Development and Utilization of Toner Materialsxe2x80x9d published by Japan Scientific Information Co., Ltd., page 121, there has been described xe2x80x9cWith extensive development of printers such as ICP, a high image quality has been required. In particular, it has been demanded to develop high-precision or high-definition printers. In Table 1, there is shown a relationship between definitions obtained by using the respective toners. As is apparent from Table 1, the smaller the particle size of wet toners, the higher the image definition is obtained. Therefore, when a dry toner is used, in order to enhance the image definition, it is also required to reduce the particle size of the toner . . . As reports of using toners having a small particle size, it has been proposed that by using toners having a particle size of 8.5 to 11 xcexcm, fogs on a background can be improved and toner consumption can be reduced, and further by using polyester-based toners having a particle size of 6 to 10 xcexcm, an image quality, a charging stability and lifetime of the developer can be improved. However, when such toners having a small particle size are used, it has been required to solve many problems. There are problems such as improvement in productivity, sharpness of particle size distribution, improvement in fluidity, etc.xe2x80x9d.
Further, black magnetic toners widely used at the present time, have been required to show a high degree of blackness and a high image density for line images and solid area images on copies.
With respect to this fact, on page 272 of the above-mentioned xe2x80x9cComprehensive Data Collection for Development and Utilization of Toner Materialsxe2x80x9d, there has been described xe2x80x9cPowder development is characterized by a high image density. However, the high image density as well as the fog density as described hereinafter, greatly influences image characteristics obtainedxe2x80x9d.
There is a close relationship between properties of the magnetic toner and those of the magnetic particles mixed and dispersed in the magnetic toner.
That is, the fluidity of the magnetic toner is largely varied depending upon surface condition of the magnetic particles exposed to the surface of the magnetic toner. Therefore, the magnetic particles themselves have been strongly required to show an excellent fluidity.
The degree of blackness and density of the magnetic toner are also largely varied depending upon the degree of blackness and density of the magnetic particles as a black pigment contained in the magnetic toner.
As the black pigment, magnetite particles have been widely used from the standpoints of magnetic properties such as saturation magnetization or coercive force, low price, color tone or the like. In addition to the magnetite particles, carbon black fine particles may be added. However, in the case where the carbon black fine particles are used in a large amount, the volume resistivity thereof is lowered to less than 1.0xc3x971013 xcexa9xc2x7cm, so that it is not possible to use the obtained toner as an insulated or high-resistant magnetic toner. Further, the dispersibility of the magnetite particles in the binder resin is deteriorated.
Hitherto, in order to enhance the fluidity of the black magnetic toner, there have been many attempts of improving the fluidity of the magnetite particles mixed and dispersed in the magnetic toner. For example, there have been proposed (1) a method of forming spherical-shaped magnetite particles (Japanese Patent Application Laid-Open (KOKAI) No. 59-64852(1984)), (2) a method of exposing a silicon compound to the surface of magnetite particles (Japanese Patent Publication (KOKOKU) No. 8-25747(1996)), or the like.
Black magnetic composite particles for black magnetic toner, which have not only a more excellent fluidity and blackness, but also an excellent dispersibility in a binder resin, are presently strongly demanded. However, black magnetic composite particles capable of satisfying all of these requirements have not been obtained yet.
Namely, the above-mentioned spherical magnetite particles show a higher fluidity than those of cubic magnetite particles, octahedral magnetite particles or the like. However, the fluidity of the spherical magnetite particles is still insufficient, and further the blackness is disadvantageously low.
As to the above-mentioned magnetite particles to the surface of which the silicon compound is exposed, the fluidity thereof is also still insufficient, and the blackness thereof is also disadvantageously low.
As a result of the present inventor""s earnest studies for solving the above problems, it has been found that by using as magnetic particles for a black magnetic toner, black magnetic composite particles having an average particle size of 0.06 to 1.0 xcexcm, comprising: magnetic iron oxide particles as core particles; a coating layer comprising an organosilicon compound which is formed on the surface of each magnetic iron oxide particle; and a carbon black coat formed onto at least a part of the surface of the coating layer in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles, the obtained black magnetic toner not only exhibits a more excellent fluidity and a more excellent blackness, but also has a high volume resistivity value and, therefore, can realize a high image quality and a high copying speed. The present invention has been attained on the basis of the finding.
It is an object of the present invention to provide black magnetic composite particles for high-resistant black magnetic toner, which are not only more excellent in fluidity and blackness, but also can show an excellent dispersibility in a binder resin.
It is another object of the present invention to provide a black magnetic toner which is not only more excellent in fluidity and blackness, but also can have a high volume resistivity value.
To accomplish the aims, in a first aspect of the present invention, there is provided a black magnetic toner comprising:
a binder resin, and
black magnetic composite particles having an average particle diameter of 0.06 to 1.0 xcexcm, comprising:
magnetic iron oxide particles;
a coating layer formed on the surface of the magnetic iron oxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and a carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles.
In a second aspect of the present invention, there is provided black magnetic toner comprising:
a binder resin, and
black magnetic composite particles having an average particle diameter of 0.06 to 1.0 xcexcm, comprising:
magnetic iron oxide particles;
a coat formed on at least a part of the surface of the magnetic iron oxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 50% by weight, calculated as Al or SiO2, based on the total weight of the magnetic iron oxide particles;
a coating layer formed on the surface of the magnetic iron oxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and a carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles.
In a third aspect of the present invention, there are provided black magnetic composite particles for a black magnetic toner, comprising:
magnetic iron oxide particles having an average particle diameter of 0.055 to 0.95 xcexcm;
a coating layer formed on the surface of the magnetic iron oxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and a carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles.
In a fourth aspect of the present invention, there are provided black magnetic composite particles for a black magnetic toner, comprising:
magnetic iron oxide particles having an average particle diameter of 0.055 to 0.95 xcexcm;
a coat formed on at least a part of the surface of the magnetic iron oxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 50% by weight, calculated as Al or SiO2, based on the total weight of the magnetic iron oxide particles;
a coating layer formed on the surface of the magnetic iron oxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and a carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles.
In a fifth aspect of the present invention, there is provided a process for producing black magnetic composite particles defined in the third aspect, which process comprises:
mixing as core particles magnetic iron oxide particles having an average particle size of 0.055 to 0.95 xcexcm together with at least one compound selected from the group consisting of:
(1) alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes,
by using an apparatus capable of applying a shear force to the particles, thereby coating the surface of the magnetic iron oxide particle with the said compounds;
mixing the obtained magnetic iron oxide particles coated with the said compounds and carbon black fine particles having an average particle size of 0.002 to 0.05 xcexcm in an amount of 1 to 25 parts by weight based on 100 parts by weight of the core particles by using an apparatus capable of applying a shear force to the particles, thereby forming a carbon black coat on the surface of the coating layer comprising the said compounds;
mixing the carbon black-coated magnetic iron oxide particles with dimethylpolysiloxanes in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the core particles by using an apparatus capable of applying a shear force to the particles; and
mixing the obtained magnetic iron oxide particles coated with dimethyl polysiloxanes and carbon black fine particles having an average particle size of 0.002 to 0.05 xcexcm in an amount of 1 to 30 parts by weight based on 100 parts by weight of the core particles by using an apparatus capable of applying a shear force to the particles, thereby further forming a carbon black coat through the dimethylpolysiloxanes.
The present invention is now described in detail below.
First, the black magnetic composite particles according to the present invention are described.
The black magnetic composite particles according to the present invention, comprise magnetic iron oxide particles as core particles; a coating layer comprising organosilicon compound, formed on the surface of each magnetic iron oxide particle; and a carbon black coat formed in a large amount, and have an average major axial diameter of 0.06 to 1.0 xcexcm.
As the magnetic iron oxide particles used as core particles in the present invention, there may be exemplified magnetite particles (FeOx.Fe2O3; 0 less than Xxe2x89xa61), maghemite particles (xcex3-Fe2O3) or a mixture of these particles. In the consideration of the blackness of the obtained black magnetic composite particles, the magnetite particles are preferred.
As the magnetic iron oxide particles as core particles, from the viewpoint of a particle shape thereof, there may be exemplified isotropic particles having a sphericity (ratio of an average particle length to an average particle breadth; hereinafter referred to merely as xe2x80x9csphericityxe2x80x9d) of usually not less than 1.0 and less than 2.0, such as spherical particles, granular particles or polyhedral particles, e.g., hexahedral particles or octahedral particles, or anisotropic particles having an aspect ratio (ratio of an average major axial diameter to an average minor axial diameter; hereinafter referred to merely as xe2x80x9caspect ratioxe2x80x9d) of not less than 2:1, such as acicular particles, spindle-shaped particles or rice ball-shaped particles. In the consideration of the fluidity of the obtained black magnetic composite particles, the magnetic iron oxide particles having an isotropic shape are preferred. Among them, the spherical particles are more preferred.
In the case of the isotropic magnetic iron oxide particles, the average particle size (diameter) thereof is 0.055 to 0.95 xcexcm, preferably 0.065 to 0.75 xcexcm, more preferably 0.065 to 0.45 xcexcm. The sphericity thereof is usually not less than 1.0:1 and less than 2.0:1, preferably 1.0:1 to 1.8:1, and in case where the shape of the magnetic iron oxide particles is spherical, the sphericity thereof is preferably 1.0:1 to 1.4:1, more preferably 1.0:1 to 1.3:1.
In the case of the anisotropic magnetic iron oxide particles, the average major axial diameter thereof is 0.055 to 0.95 xcexcm, preferably 0.065 to 0.75 xcexcm, more preferably 0.065 to 0.45 xcexcm, and the aspect ratio thereof is 2:1 to 20:1, preferably 2:1 to 18:1, more preferably 2:1 to 15:1.
When the average particle size of the magnetic iron oxide particles is more than 0.95 xcexcm, the obtained black magnetic composite particles are coarse particles and are deteriorated in tinting strength. On the other hand, when the average particle size is less than 0.055 xcexcm, the intermolecular force between the particles is increased due to the reduction in particle size (fine particle), so that agglomeration of the particles tends to be caused. As a result, it becomes difficult to uniformly coat the surfaces of the magnetic iron oxide particles with the organosilicon compounds, and uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds.
Further, in the case where the upper limit of the aspect ratio of the anisotropic magnetic iron oxide particles exceeds 20:1, the particles tend to be entangled with each other, and it also becomes difficult to uniformly coat the surfaces of the magnetic iron oxide particles with the organosilicon compounds, and uniformly form the carbon black coat on the surface of the coating layer composed of the organosilicon compounds.
As to the particle size distribution of the magnetic iron oxide particles, the geometrical standard deviation value thereof is preferably not more than 2.0, more preferably not more than 1.8, still more preferably not more than 1.6. When the geometrical standard deviation value thereof is more than 2.0, coarse particles are contained therein, so that the particles are inhibited from being uniformly dispersed. As a result, it also becomes difficult to uniformly coat the surfaces of the magnetic iron oxide particles with the organosilicon compounds, and uniformly form the carbon black coat on the surface of the coating layer composed of the organosilicon compounds. The lower limit of the geometrical standard deviation value is 1.01. It is industrially difficult to obtain particles having a geometrical standard deviation value of less than 1.01.
The BET specific surface area of the magnetic iron oxide particles thereof is not less than 0.5 m2/g. When the BET specific surface area is less than 0.5 m2/g, the magnetic iron oxide particles may become coarse particles, or the sintering between the particles may be caused, so that the obtained black magnetic composite particles also may become coarse particles and tend to be deteriorated in tinting strength. In the consideration of the tinting strength of the obtained black magnetic composite particles, the BET specific surface area of the magnetic iron oxide particles is preferably not less than 1.0 m2/g, more preferably 1.5 m2/g. Further, in the consideration of uniformly coating the surfaces of the magnetic iron oxide particles with the organosilicon compounds, and uniformly forming the carbon black coat on the coating layer composed of the organosilicon compounds, the upper limit of the BET specific surface area of the magnetic iron oxide particles, is usually 95 m2/g, preferably 90 m2/g, more preferably 85 m2/g.
As to the fluidity of the magnetic iron oxide particles, the fluidity index thereof is about 25 to about 43. Among the magnetic iron oxide particles having various shapes, the spherical particles are excellent in fluidity, for example, the fluidity index thereof is about 30 to about 43.
As to the blackness of the magnetic iron oxide particles, in the case of the magnetite particles, the lower limit thereof is usually 18.0 when represented by L* value, and the upper limit thereof is usually 26.0, preferably 25.0 when represented by L* value. In the case of maghemite particles, the lower limit thereof is usually more than 18.0 when represented by L* value, and the upper limit thereof is usually 34.0, preferably 32.0 when represented by L* value. When the L* value exceeds the above-mentioned upper limit, the lightness of the particles is increased, so that it is difficult to obtain black magnetic composite particles having a sufficient blackness.
As to the magnetic properties of the magnetic iron oxide particles, the coercive force value thereof is usually 0.8 to 31.8 kA/m (10 to 400 Oe), preferably 1.6 to 30.2 kA/m (20 to 380 Oe); the saturation magnetization value in a magnetic field of kA/m (10 kOe) is usually 50 to 91 Am2/kg (50 to 91 emu/g), preferably 60 to 90 Am2/kg (60 to 90 emu/g); and the residual magnetization value in a magnetic field of 795.8 kA/m (10 kOe) is usually 1 to 35 Am2/kg (1 to 35 emu/g), preferably 3 to 30 Am2/kg (3 to 30 emu/g).
As the core particles, there may be used magnetic iron oxide particles wherein at least a part of magnetic iron oxide particles is preliminarily coated with at least one compound selected from the group consisting of hydroxide of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon (hereinafter referred to as xe2x80x9chydroxides and/or oxides of aluminum and/or siliconxe2x80x9d) In this case, the dispersibility of the obtained composite particles in a vehicle may become improved as compared to those having no undercoat composed of hydroxides and/or oxides of aluminum and/or silicon, because the percentage of desorption of carbon black from the non-magnetic acicular black iron-based composite particles is lessened.
The amount of the hydroxides and/or oxides of aluminum and/or silicon coat is 0.01 to 50% by weight calculated as Al, SiO2 or a sum of Al and SiO2, based on the weight of the magnetic iron oxide particles as the core particles.
When the amount of the hydroxides and/or oxides of aluminum and/or silicon coat is less than 0.01% by weight; the improvement of the dispersibility of the obtained black magnetic composite particles in a vehicle cannot be achieved because of failing to achieve the improvement of lessening the percentage of desorption of carbon black therefrom. On the other hand, when the amount of the hydroxides and/or oxides of aluminum and/or silicon coat is more than 50% by weight, the obtained black magnetic composite particles can exhibit a good dispersibility in a vehicle by the improvement of lessening the percentage of desorption of carbon black therefrom, but the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the hydroxides and/or oxides of aluminum and/or silicon coat.
The black magnetic composite particles using as core particles the magnetic iron oxide particles having the coat composed of the hydroxides and/or oxides of aluminum and/or silicon may be substantially identical in a particle size, a geometrical standard deviation, a BET specific surface area, a blackness (L* value), a fluidity and a magnetic property, to those having no hydroxides and/or oxides of aluminum and/or silicon coat.
The particle shape and particle size of the black magnetic composite particles according to the present invention are considerably varied depending upon those of the magnetic iron oxide particles as core particles. The black magnetic composite particles have a similar particle shape to that of the magnetic iron oxide particle as core particle, and a slightly larger particle size than that of the magnetic iron oxide particles as core particles.
More specifically, when the isotropic magnetic iron oxide particles are used as core particles, the obtained black magnetic composite particles according to the present invention, have an average particle size of usually 0.06 to 1.0 xcexcm, preferably 0.07 to 0.8 xcexcm, more preferably 0.07 to 0.5 xcexcm and a sphericity of usually not less than 1.0:1 and less than 2.0:1, preferably 1.0:1 to 1.8:1, and in case where the shape of the magnetic iron oxide particles is spherical, the sphericity thereof is preferably 1.0:1 to 1.4:1, more preferably 1.0:1 to 1.3:1.
When the anisotropic magnetic iron oxide particles are used as core particles, the obtained black magnetic composite particles according to the present invention, have an average particle size of usually 0.06 to 1.0 xcexcm, preferably 0.07 to 0.8 xcexcm, more preferably 0.07 to 0.5 xcexcm and an aspect ratio of usually 2:1 to 20:1, preferably 2.5:1 to 18:1, more preferably 2:1 to 15:1.
When the average particle size of the black magnetic composite particles is more than 1.0 xcexcm, the obtained black magnetic composite particles may be coarse particles, and deteriorated in tinting strength. On the other hand, when the average particle size thereof is less than 0.06 xcexcm, the black magnetic composite particles may tend to be agglomerated by the increase of intermolecular force due to the reduction in particle size, thereby deteriorating the dispersibility in a binder resin upon production of the magnetic toner.
When the aspect ratio is more than 20:1, the black magnetic composite particles may be entangled with each other in the binder resin, so that the dispersibility in binder resin may tend to be deteriorated.
The geometrical standard deviation value of the black magnetic composite particles according to the present invention is preferably not more than 2.0, more preferably not more than 1.8, still more preferably not more than 1.6. The lower limit of the geometrical standard deviation value thereof is preferably 1.01. When the geometrical standard deviation value thereof is more than 2.0, the tinting strength of the black magnetic composite particles may be likely to be deteriorated due to the existence of coarse particles therein. It is industrially difficult to obtain such particles having a geometrical standard deviation of less than 1.01.
The BET specific surface area of the black magnetic composite particles according to the present invention, is usually 1.0 to 100 m2/g, preferably 1.5 to 95 m2/g, more preferably 2.0 to 90 m2/g. When the BET specific surface area thereof is less than 1.0 m2/g, the obtained black magnetic composite particles may be coarse, and the sintering between the black magnetic composite particles may be caused, thereby deteriorating the tinting strength. On the other hand, when the BET specific surface area is more than 100 m2/g, the black magnetic composite particles may tend to be agglomerated together by the increase in intermolecular force due to the reduction in particle size, thereby deteriorating the dispersibility in a binder resin upon production of the magnetic toner.
As to the fluidity of the black magnetic composite particles according to the present invention, the fluidity index thereof is preferably 48 to 90, more preferably 49 to 90, still more preferably 50 to 90. When the fluidity index thereof is less than 48, the fluidity of the black magnetic composite particles may become insufficient, thereby failing to improve the fluidity of the finally obtained magnetic toner. Further, in the production process of the magnetic toner, there may tend to be caused defects such as clogging of hopper, etc., thereby deteriorating the handling property or workability.
As to the blackness of the black magnetic composite particles according to the present invention, in the case magnetite particles are used as core particles, the upper limit of the blackness of the black magnetic composite particles is usually 19.5, preferably 18.8, more preferably 17.8 when represented by L* value. In the case maghemite particles are used as core particles, the upper limit of the blackness of the black magnetic composite particles is usually 19.5, preferably 19.0, more preferably 18.8 when represented by L* value. When the L* value thereof is more than 19.5, the lightness of the obtained black magnetic composite particles may become high, so that the black magnetic composite particles having a sufficient blackness may not be obtained. The lower limit of the blackness thereof is 15 when represented by L* value.
The dispersibility in binder resin of the black magnetic composite particles according to the present invention, is preferably 4th or 5th rank, more preferably 5th rank when evaluated by the method described hereinafter.
The percentage of desorption of carbon black from the black magnetic composite particles according to the present invention, is preferably not more than 20%, more preferably not more than 10%. When the desorption percentage of the carbon black is more than 20%, the desorbed carbon black may tend to inhibit the black magnetic composite particles from being uniformly dispersed in the binder resin upon production of the magnetic toner.
The magnetic properties of the black magnetic composite particles according to the present invention, can be controlled by appropriately selecting kind and particle shape of the magnetic iron oxide particles as core particles. Similarly to magnetic properties of magnetic particles ordinarily used for the production of magnetic toner, the coercive force of the black magnetic composite particles according to the present invention, is usually 0.8 to 31.8 kA/m (10 to 400 Oe), preferably 1.6 to 30.2 kA/m (20 to 380 Oe); the saturation magnetization in a magnetic field of 795.8 kA/m (10 kOe) is usually 50 to 91 Am2/kg (50 to 91 emu/g), preferably 60 to 90 Am2/kg (60 to 90 emu/g); and the residual magnetization in a magnetic field of 795.8 kA/m (10 kOe) is usually 1 to 35 Am2/kg (1 to 35 emu/g), preferably 3 to 30 Am2/kg (3 to 30 emu/g).
The coating layer formed on the surfaces of the core particles comprises at least one organosilicon compound selected from the group consisting of (1) organosilane compounds obtainable from alkoxysilane compounds; and (2) polysiloxanes, or (2xe2x80x2) modified polysiloxanes selected from the group consisting of (A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds (hereinafter referred to merely as xe2x80x9cmodified polysiloxanesxe2x80x9d), and (B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group (hereinafter referred to merely as xe2x80x9cterminal-modified polysiloxanesxe2x80x9d).
The organosilane compounds (1) may be produced by drying or heat-treating alkoxysilane compounds represented by the formula (I):
R1aSiX4xe2x88x92axe2x80x83xe2x80x83(I)
wherein R1 is C6H5xe2x80x94, (CH3)2CHCH2xe2x80x94 or n-CbH2b+1xe2x80x94 (wherein b is an integer of 1 to 18); X is CH3Oxe2x80x94 or C2H5Oxe2x80x94; and a is an integer of 0 to 3.
The drying or heat-treatment of the alkoxysilane compounds may be conducted, for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
Specific examples of the alkoxysilane compounds may include methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethyoxysilane, diphenyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane or the like. Among these alkoxysilane compounds, in view of the desorption percentage and the adhering effect of carbon black, methyltriethoxysilane, phenyltriethyoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane and isobutyltrimethoxysilane are preferred, and methyltriethoxysilane and methyltrimethoxysilane are more preferred.
As the polysiloxanes (2), there may be used those compounds represented by the formula (II): 
wherein R2 is Hxe2x80x94 or CH3xe2x80x94, and d is an integer of 15 to 450.
Among these polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, polysiloxanes having methyl hydrogen siloxane units are preferred.
As the modified polysiloxanes (2xe2x80x2-A), there may be used:
(a1) polysiloxanes modified with polyethers represented by the formula (III): 
xe2x80x83wherein R3 is xe2x80x94(xe2x80x94CH2xe2x80x94)hxe2x80x94; R4 is xe2x80x94(xe2x80x94CH2xe2x80x94)ixe2x80x94CH3; R5 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94C(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)jxe2x80x94CH3; R6 is xe2x80x94(xe2x80x94CH2xe2x80x94)kxe2x80x94CH3; g and h are an integer of 1 to 15; i, j and k are an integer of 0 to 15; e is an integer of 1 to 50; and f is an integer of 1 to 300;
(a2) polysiloxanes modified with polyesters represented by the formula (IV): 
xe2x80x83wherein R7, R8 and R9 are xe2x80x94(xe2x80x94CH2xe2x80x94)qxe2x80x94 and may be the same or different; R10 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94C(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)rxe2x80x94CH3; R11 is xe2x80x94(xe2x80x94CH2xe2x80x94)sxe2x80x94CH3; n and q are an integer of 1 to 15; r and s are an integer of 0 to 15; exe2x80x2 is an integer of 1 to 50; and fxe2x80x2 is an integer of 1 to 300;
(a3) polysiloxanes modified with epoxy compounds represented by the formula (V): 
xe2x80x83wherein R12 is xe2x80x94(xe2x80x94CH2xe2x80x94)vxe2x80x94; v is an integer of 1 to 15; t is an integer of 1 to 50; and u is an integer of 1 to 300; or a mixture thereof.
Among these modified polysiloxanes (2xe2x80x2-A), in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes modified with the polyethers represented by the formula (III), are preferred.
As the terminal-modified polysiloxanes (2xe2x80x2-B), there may be used those represented by the formula (VI): 
wherein R13 and R14 are xe2x80x94OH, R16OH or R17COOH and may be the same or different; R15 is xe2x80x94CH3 or xe2x80x94C6H5; R16 and R17 are xe2x80x94(xe2x80x94CH2xe2x80x94)yxe2x80x94; y is an integer of 1 to 15; w is an integer of 1 to 200; and x is an integer of 0 to 100.
Among these terminal-modified polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes whose terminals are modified with carboxylic acid groups are preferred.
The amount of the coating layer composed of the organosilicon compounds is usually 0.02 to 5.0% by weight, preferably 0.03 to 4.0% by weight, more preferably 0.05 to 3.0% by weight (calculated as Si) based on the weight of the magnetic iron oxide particles coated with the organosilicon compounds.
When amount of the coating layer composed of the organosilicon compounds is less than 0.02% by weight, it becomes difficult to adhere the carbon black on the surfaces of the magnetic iron oxide particles. On the other hand, in case where the coating amount of the organosilicon compounds is more than 5.0% by weight, since the carbon black coat can be sufficiently formed on the surface of the coating layer composed of the organosilicon compounds, it is meaingless to coat an excess amount of the organosilicon compounds.
A carbon black coat is formed on at least a part of the surface of coating layer composed of the organosilicon compounds, and is composed of at least two carbon black layers integrally adhered with each other through an adhesive. If required, 3 or more carbon black layers are integrally adhered with each other through an adhesive to form the carbon black coat.
The amount of the carbon black coat is 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles as core particles.
When the amount of the carbon black coat formed is less than 26 part by weight, it becomes difficult to obtain black magnetic composite particles having a sufficient fluidity and blackness. On the other hand, when the amount of the carbon black coat formed is more than 55 parts by weight, the carbon black tend to be desorbed from the coating layer composed of the organosilicon compound. As a result, the obtained black magnetic composite particles tend to be deteriorated in dispersibility in a binder resin upon the production of magnetic toner.
The thickness of carbon black coat formed is preferably not more than 0.06 xcexcm, more preferably not more than 0.05 xcexcm, still more preferably not more than 0.04 xcexcm. The lower limit thereof is more preferably 0.0001 xcexcm.
In the black magnetic composite particles according to the present invention, at least a part of the surface of the magnetic iron oxide particles as core particle may be preliminarily coated with hydroxides and/or oxides of aluminum and/or silicon. In this case, the obtained black magnetic composite particles can show a higher dispersibility in a binder resin as compared to in the case where the magnetic iron oxide particles are uncoated with hydroxides and/or oxides of aluminum and/or silicon, because of achieving the improvement of lessening the percentage of desorption of carbon black therefrom.
By coating the magnetic iron oxide particle with the hydroxides and/or oxides of aluminum and/or silicon, the percentage of desorption of carbon black from the obtained black magnetic composite particles of the present invention is preferably not more than 10%, more preferably not more than 5%.
Next, the black magnetic toner according to the present invention is described.
The black magnetic toner according to the present invention comprises the black magnetic composite particles, and a binder resin. The black magnetic toner may further contain a mold release agent, a colorant, a charge-controlling agent and other additives, if necessary.
The black magnetic toner according to the present invention has an average particle size of usually 3 to 15 xcexcm, preferably 5 to 12 xcexcm.
The amount of the binder resin used in the black magnetic toner is usually 50 to 900 parts by weight, preferably 50 to 400 parts by weight based on 100 parts by weight of the black magnetic composite particles.
As the binder resins, there may be used vinyl-based polymers, i.e., homopolymers or copolymers of vinyl-based monomers such as styrene, alkyl acrylates and alkyl methacrylates. As the styrene monomers, there may be exemplified styrene and substituted styrenes. As the alkyl acrylate monomers, there may be exemplified acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate or the like.
It is preferred that the above copolymers contain styrene-based components in an amount of usually 50 to 95% by weight.
In the binder resin used in the present invention, the above-mentioned vinyl-based polymers may be used in combination with polyester-based resins, epoxy-based resins, polyurethane-based resins or the like, if necessary.
As to the fluidity of the black magnetic toner according to the present invention, the fluidity index is usually 78 to 100, preferably 79 to 100, more preferably 80 to 100. When the fluidity index is less than 78, the black magnetic toner may not show a sufficient fluidity.
The blackness of the black magnetic toner according to the present invention is usually not more than 19.0, preferably not more than 18.8, more preferably not more than 18.5 when represented by L* value. When the blackness thereof is more than 19.0, the lightness of the black magnetic toner may be increased, resulting in insufficient blackness. The lower limit of the blackness of the black magnetic toner is usually about 15 when represented by L* value.
The volume resistivity of the black magnetic toner according to the present invention, is usually not less than 1.0xc3x971013 xcexa9xc2x7cm, preferably not less than 3.0xc3x971013 xcexa9xc2x7cm, more preferably not less than 5.0xc3x971013 xcexa9xc2x7cm. When the volume resistivity is less than 1.0xc3x971013 xcexa9xc2x7cm, the charge amount of the black magnetic toner tends to vary depending upon environmental conditions in which the toner is used, resulting in unstable properties of the black magnetic toner. The upper limit of the volume resistivity is 1.0xc3x971017 xcexa9xc2x7cm.
As to the magnetic properties of the black magnetic toner according to the present invention, the coercive force thereof is usually 0.8 to 31.8 kA/m (10 to 400 Oe), preferably 1.6 to 30.2 kA/m (20 to 380 Oe); the saturation magnetization value in a magnetic field of 795.8 kA/m (10 kOe) is usually 10 to 85 Am2/kg (10 to 85 emu/g), preferably 20 to 80 Am2/kg (20 to 80 emu/g); the residual magnetization in a magnetic field of 795.8 kA/m (10 kOe) is usually 1 to 20 Am2/kg (1 to 20 emu/g), preferably Am2/kg (2 to 15 emu/g; the saturation magnetization in a magnetic field of 79.6 kA/m (1 kOe) is usually 7.5 to 65 Am2/kg (7.5 to 65 emu/g), preferably 10 to 60 Am2/kg (10 to 60 emu/g); and the residual magnetization in a magnetic field of 79.6 kA/m (1 kOe) is usually 0.5 to 15 Am2/kg (0.5 to 15 emu/g), preferably 1.0 to 13 Am2/kg (1.0 to 13 emu/g).
Next a process for producing the black magnetic composite particles according to the present invention is described.
Among the isotropic magnetite particles which are magnetic iron oxide particles, (1) octahedral magnetite particles can be produced by passing an oxygen-containing gas through a suspension containing ferrous hydroxide colloid having a pH value of not less than 10, which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution having a concentration of not less than one equivalent based on Fe2+ in the aqueous ferrous salt solution, thereby precipitating magnetite particles, and then subjecting the obtained magnetite particles to filtering, washing with water and drying (Japanese Patent Publication (KOKOKU) No. 44-668(1969); (2) hexahedral magnetite particles can be produced by passing an oxygen-containing gas through a suspension containing ferrous hydroxide colloid having a pH value of 6.0 to 7.5, which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution having a concentration of not more than one equivalent based on Fe2+ in the aqueous ferrous salt solution to produce magnetite core particles, further passing an oxygen-containing gas through the obtained aqueous ferrous salt reaction solution containing the magnetite core particles and the ferrous hydroxide colloid, at a pH value of 8.0 to 9.5, to precipitate magnetite particles, and then subjecting the precipitated magnetite particles to filtering, washing with water and drying (Japanese Patent Application Laid-Open (KOKAI) No. 3-201509(1991); (3) spherical magnetite particles can be produced by passing an oxygen-containing gas through a suspension containing ferrous hydroxide colloid having a pH value of 6.0 to 7.5, which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution having a concentration of not more than one equivalent based on Fe2+ in the aqueous ferrous salt solution to produce magnetite core particles, adding alkali hydroxide in an amount of not less than equivalent based on the remaining Fe2+ to adjust the pH value of the suspension to not less than 10, heat-oxidizing the resultant suspension to precipitate magnetite particles, and then subjecting the precipitated magnetite particles to filtering, washing with water and drying (Japanese Patent Publication (KOKOKU) No. 62-51208(1987).
The isotropic maghemite particles can be obtained by heating the above-mentioned isotropic magnetite particles in air at 300 to 600xc2x0 C.
The anisotropic magnetite particles can be produced by passing an oxygen-containing gas through a suspension containing either ferrous hydroxide colloid, iron carbonate, or an iron-containing precipitate obtained by reacting an aqueous ferrous salt solution with alkali hydroxide and/or alkali carbonate, while appropriately controlling the pH value and temperature of the suspension, to produce acicular, spindle-shaped or rice ball-shaped goethite particles, subjecting the obtained goethite particles to filtering, washing with water and drying, and then reducing the goethite particles in a heat-reducing gas at 300 to 800xc2x0 C.
The anisotropic maghemite particles can be produced by heat-oxidizing the above-mentioned anisotropic magnetite particles in an oxygen-containing gas at 300 to 600xc2x0 C.
The coating of the magnetic iron oxide particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, may be conducted (i) by mechanically mixing and stirring the magnetic iron oxide particles together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes; or (ii) by mechanically mixing and stirring both the components together while spraying the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes onto the magnetic iron oxide particles. In these cases, substantially whole amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added can be applied onto the surfaces of the magnetic iron oxide particles.
In order to uniformly coat the surfaces of the magnetic iron oxide particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, it is preferred that the magnetic iron oxide particles are preliminarily diaggregated by using a pulverizer.
As apparatuses used for (a) mixing and stirring the core particles with alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes; (b) mixing and stirring the carbon black fine particles with the particles surface-coated with alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes; (c) mixing and stirring the adhesive with the particles having a first carbon black coat formed onto the surface-coating composed of alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes (hereinafter referred to as xe2x80x9ccomposite particlesxe2x80x9d); and (d) mixing and stirring the carbon black fine particles with the composite particles coated with the adhesive, there may be preferably used apparatus capable of applying a shearing force to a layer of the particles to be treated, more preferably those capable of conducting shearing, spatula-stroking and compression at the same time, for example, wheel-type kneader, ball-type kneader, blade-type kneader, roll-type kneader or the like. Among these apparatuses, the wheel-type kneader is more effective for the practice of the present invention.
Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller, or the like. Among them, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred.
Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include an extruder or the like.
In order to coat the surfaces of the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment may be appropriately controlled such that the linear load is usually 19.6 to 1960 N/cm (2 to 200 Kg/cm), preferably 98 to 1470 N/cm (10 to 150 kg/cm), more preferably 147 to 980 N/cm (15 to 100 kg/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added, is preferably 0.15 to 45 parts by weight based on 100 parts by weight of the magnetic iron oxide particles. When the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added is less than 0.15 part by weight, it may become difficult to form the carbon black coat on the coating layer.
On the other hand, when the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added is more than 45 parts by weight, a sufficient amount of the carbon black coat can be formed on the surface of the coating, and therefore, it is meaningless to add such an excess amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes.
Meanwhile, a part of the alkoxysilanes coated on the surfaces of the core particles may be converted into the organosilane compounds via the coating step thereof. Even in such a case, the subsequent adhesion step with carbon black is not adversely affected.
Next, the carbon black fine particles are added to the magnetic iron oxide particles coated with the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, and the resultant mixture is mixed and stirred to form a first carbon black coat on the surfaces of the coating composed of the the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added.
In order to form carbon black coat onto the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment can be appropriately controlled such that the linear load is usually 2 to 200 Kg/cm, preferably 10 to 150 Kg/cm more preferably 15 to 100 Kg/cm; and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the carbon black fine particles added for forming the first carbon black coat, is usually 1 to 25 parts by weight, preferably 5 to 25 parts by weight based on 100 parts by weight of the magnetic iron oxide particles. When the amount of carbon black fine particles added for forming the first carbon black coat is less than 1 part by weight, the amount of the adhesive capable of adhering onto the first carbon black coat also may become insufficient. As a result, when carbon black fine particles for forming a second carbon black coat are subsequently added such that the total amount of carbon black adhered is not less than 26 parts by weight based on 100 parts by weight of the magnetic iron oxide particles as core particles, the desorption percentage of carbon black is disadvantageously increased, resulting in deteriorated dispersibility in a binder resin upon production of the magnetic toner.
On the contrary, when the amount of carbon black adhered is as large as more than 25 parts by weight, the carbon black tends to be desorbed or fallen-off from the surface of each composite particle. Therefore, the carbon black also tends to be desorbed or fallen-off from the surfaces of the obtained black magnetic composite particles, resulting in deteriorated dispersibility in a binder resin upon production of the magnetic toner.
As the carbon black fine particles used in the present invention, there may be exemplified commercially available carbon blacks such as furnace black, channel black or the like. Specific examples of the commercially available carbon blacks usable in the present invention, may include #3050, #3150, #3250, #3750, #3950, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860 ULTRA, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY), Ketchen black EC, Ketchen black EC600JD, etc. (tradename, produced by KETCHEN BLACK INTERNATIONAL CO., LTD.), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, VULCAN XC72, REGAL 660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTY CHEMICALS INK CO., LTD.), or the like.
In order to uniformly form the carbon black coat onto the coating composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, or the dimethylpolysiloxane coating layer while suppressing the reduction of volume resistivity value of the obtained composite particles, the use of carbon black fine particles having a DBP oil absorption of not more than 150 ml/100 g is preferred. Specific examples of the commercially available carbon blacks usable in the present invention, may include MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860 ULTRA, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, REGAL 660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTY CHEMICALS INK CO., LTD.).
The average particle size of the carbon black fine particles used is usually 0.002 to 0.05 xcexcm, preferably 0.002 to 0.035 xcexcm. When the average particle size of the carbon black fine particles used is less than 0.002 xcexcm, the carbon black fine particles used are too fine to be well handled.
On the other hand, when the average particle size thereof is more than 0.05 xcexcm, since the particle size of the carbon black fine particles used is much larger, it is necessary to apply a larger mechanical shear force for forming the uniform carbon black coat on the coating layer composed of the organosilicon compounds, thereby rendering the coating process industrially disadvantageous.
It is preferred that the carbon black fine particles are added little by little and slowly, especially about 5 to 60 minutes.
In order to form the first carbon black coat onto the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment can be appropriately controlled such that the linear load is usually 19.6 to 1960 N/cm (2 to 200 Kg/cm), preferably 98 to 1470 N/cm (10 to 150 Kg/cm), more preferably 147 to 980 N/cm (15 to 100 Kg/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
Then, a second carbon black coat is formed onto the first carbon black coat through an adhesive such as dimethylpolysiloxanes.
The first and second carbon black coats can be bonded to each other by adhering the carbon black themselves through the adhesive, thereby obtaining a carbon black coat wherein the first and second carbon black coats are integrated. In order to obtain black magnetic composite particles exhibiting excellent fluidity and blackness, in which the two carbon black coats are firmly and uniformly bonded together, dimethylpolysiloxanes represented by the following formula is preferably used as the adhesive. 
wherein vxe2x80x2 is a is an integer of 15 to 450.
The amount of the adhesive added is usually 0.1 to 5.0 parts by weight, preferably 0.2 to 4.0 parts by weight, more preferably 0.3 to 3.0 parts by weight based on 100 parts by weight of the core particles.
When the amount of the adhesive adhered is less than 0.1 part by weight, it may be difficult to sufficiently bond the second carbon black coat onto the first carbon black coat, thereby failing to obtain black magnetic composite particles exhibiting a more excellent fluidity and a more excellent blackness.
When the amount of the adhesive adhered is more than 5.0 part by weight, the carbon black can be adhered thereon in such an amount enough to achieve the more excellent fluidity and blackness of the obtained black magnetic composite particles. However, since the effect is already saturated, it is unnecessary to use such a large amount of the adhesive.
The amount of the adhesive is usually 0.04 to 1.89% by weight, preferably 0.08 to 1.51% by weight, more preferably 0.11 to 1.13% by weight (calculated as Si) based on the weight of the magnetic iron oxide particles.
After the adhesive is added to, and then mixed and stirred with the composite particles on which the first carbon black coat is formed, the carbon black fine particles are added to, and then mixed and stirred with the resultant mixture to form the second carbon black coat onto the first carbon black coat through the adhesive, thereby integrating the carbon black coats. The thus obtained composite particles may be dried and heat-treated, if required.
The mixing and stirring conditions for adhering the adhesive onto the composite particles on which the first carbon black coat is formed, may be appropriately selected such that the adhesive can be uniformly coated onto the first carbon black coat of each composite particle. More specifically, the linear load used for the mixing and stirring is usually 19.6 to 1,960 N/cm (2 to 200 kg/cm), preferably 98 to 1,470 N/cm (10 to 150 kg/cm), more preferably 147 to 980 N/cm (15 to 100 kg/cm); the treating time is 5 to 120 minutes, preferably 10 to 90 minutes; and the stirring speed is usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the carbon black fine particles added for forming the second carbon black coat is 1 to 30 parts by weight based on 100 parts by weight of the magnetic iron oxide particles. When the amount of the carbon black fine particles added is less than 1 part by weight, the total amount of carbon black adhered becomes insufficient, so that it may be difficult to obtain aimed black magnetic composite particles which are more excellent in fluidity and blackness. On the contrary, when the amount of the carbon black fine particles added is more than 30 part by weight, the carbon black tends to be desorbed or fallen-off from the surfaces of the obtained black magnetic composite particles, resulting in deteriorated dispersibility in a binder resin upon production of the magnetic toner.
The mixing and stirring conditions for forming the second carbon black coat onto the first carbon black coat through the adhesive, may be appropriately selected such that the second carbon black coat can be uniformly coated onto the first carbon black coat through the adhesive. More specifically, the linear load used for the mixing and stirring is usually 19.6 to 1,960 N/cm (2 to 200 kg/cm), preferably 98 to 1,470 N/cm (10 to 150 kg/cm), more preferably 147 to 980 N/cm (15 to 100 kg/cm); the treating time is 5 to 120 minutes, preferably 10 to 90 minutes; and the stirring speed is usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
In the case where the alkoxysilane compounds are used as the coating compound, the resultant black magnetic composite particles may be dried or heat-treated, for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
At least a part of the surface of the magnetic iron oxide particles as core particles may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, in advance of mixing and stirring with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes.
The coat of the hydroxides and/or oxides of aluminum and/or silicon may be conducted by adding an aluminum compound, a silicon compound or both the compounds to a water suspension in which the magnetic iron oxide particles are dispersed, followed by mixing and stirring, and further adjusting the pH value of the suspension, if required, thereby coating the surfaces of the magnetic iron oxide particles with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The thus obtained particles coated with the hydroxides and/or oxides of aluminum and/or silicon are then filtered out, washed with water, dried and pulverized. Further, the particles coated with the hydroxides and/or oxides of aluminum and/or silicon may be subjected to post-treatments such as deaeration treatment and compaction treatment.
As the aluminum compounds, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate or the like.
The amount of the aluminum compound added is 0.01 to 50% by weight (calculated as Al) based on the weight of the magnetic iron oxide particles. When the amount of the aluminum compound added is less than 0.01% by weight., it may be difficult to sufficiently coat the surfaces of the magnetic iron oxide particles with hydroxides and/or oxides of aluminum, which can achieve the improvement of lessening the percentage of desorption of carbon black therefrom, thereby failing to achieve the improvement of the dispersibility in the binder resin upon the production of the magnetic toner. On the other hand, when the amount of the aluminum compound added is more than 50% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the aluminum compound.
As the silicon compounds, there may be exemplified water glass #3, sodium orthosilicate, sodium metasilicate, colloidal silica or the like.
The amount of the silicon compound added is 0.01 to 50% by weight (calculated as SiO2) based on the weight of the magnetic iron oxide particles. When the amount of the silicon compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the magnetic iron oxide particles with hydroxides and/or oxides of silicon, which can achieve the improvement of lessening the percentage of desorption of carbon black therefrom, thereby failing to achieve the improvement of the dispersibility in the binder resin upon the production of the magnetic toner. On the other hand, when the amount of the silicon compound added is more than 50% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the silicon compound.
In the case where both the aluminum and silicon compounds are used in combination for the coating, the total amount of the aluminum and silicon compounds added is preferably 0.01 to 50% by weight (calculated as a sum of Al and SiO2) based on the weight of the magnetic iron oxide particles.
The black magnetic toner according to the present invention may be produced by a known method of mixing and kneading a predetermined amount of a binder resin and a predetermined amount of the black magnetic composite particles together, and then pulverizing the mixed and kneaded material into particles. More specifically, the black magnetic composite particles and the binder resin are intimately mixed together with, if necessary, a mold release agent, a colorant, a charge-controlling agent or other additives by using a mixer. The obtained mixture is then melted and kneaded by a heating kneader so as to render the respective components compatible with each other, thereby dispersing the black magnetic composite particles therein. Successively, the molten mixture is cooled and solidified to obtain a resin-kneaded product. The resin-kneaded product is then pulverized and classified, thereby producing a magnetic toner having an aimed particle size.
As the mixers, there may be used a Henschel mixer, a ball mill or the like. As the heating kneaders, there may be used a roll mill, a kneader, a twin-screw extruder or the like. The pulverization of the resin mixture may be conducted by using pulverizers such as a cutter mill, a jet mill or the like. The classification of the pulverized particles may be conducted by known methods such as air classification, etc., as described in Japanese Patent No. 2683142 or the like.
As the other method of producing the black magnetic toner, there may be exemplified a suspension polymerization method or an emulsion polymerization method. In the suspension polymerization method, polymerizable monomers and the black magnetic composite particles are intimately mixed together with, if necessary, a colorant, a polymerization initiator, a cross-linking agent, a charge-controlling agent or the other additives and then the obtained mixture is dissolved and dispersed together so as to obtain a monomer composition. The obtained monomer composition is added to a water phase containing a suspension stabilizer while stirring, thereby granulating and polymerizing the composition to form magnetic toner particles having an aimed particle size.
In the emulsion polymerization method, the monomers and the black magnetic composite particles are dispersed in water together with, if necessary, a colorant, a polymerization initiator or the like and then the obtained dispersion is polymerized while adding an emulsifier thereto, thereby producing magnetic toner particles having an aimed particle size.
A point of the present invention lies in such a fact that the black magnetic composite particles according to the present invention which are obtained by firmly adhering carbon black onto the surfaces of magnetic iron oxide particles in an amount of 26 to 55 parts by weight based on 100 parts by weight of the magnetic iron oxide particles, are not only more excellent in fluidity and blackness, but also have a less amount of carbon black desorbed or fallen-off from the surface of each particle.
The reason why the black magnetic composite particles of the present invention can exhibit a more excellent fluidity, is considered as follows. In general, the carbon black tends to act as aggregates because of its fineness. In contrast, in the case of the black magnetic composite particles of the present invention, since the carbon black is uniformly and densely adhered and bonded onto the surface of each magnetic iron oxide particle, it is considered that many fine irregularities are formed on the surface of each magnetic iron oxide particle.
The reason why the black magnetic composite particles of the present invention can exhibit a more excellent blackness, is considered as follows. That is, since a uniform carbon black coat having an appropriate thickness is formed by densely adhering and bonding carbon black onto the surface of each magnetic iron oxide particle, the color of the magnetic iron oxide particles is hidden behind the carbon black coat, so that an inherent black color of the carbon black can be effectively exhibited.
The reason why the amount of the carbon black desorbed (or fallen-off) from the surfaces of the black magnetic composite particles according to the present invention, is small, is considered as follows. In the case of the alkoxysilane compounds (1) and the fluoroalkylsilane compounds (3), metalloxane bonds (xe2x89xa1Sixe2x80x94Oxe2x80x94M wherein M represents a metal atom contained in the magnetic iron oxide particles, such as Si, Al, Fe or the like) are formed between the surfaces of the magnetic iron oxide particles and alkoxy groups contained in the organosilicon compounds onto which the carbon black coat is formed, thereby forming a stronger bond between the organosilicon compounds on which the carbon black coat is formed, and the surfaces of the magnetic iron oxide particles. Further, in the case of using the polysiloxanes or modified polysiloxanes, the functional groups in the polysiloxanes or modified polysiloxanes onto which the carbon black coat is formed, are strongly bonded to the surface of the magnetic iron oxide particle.
In accordance with the present invention, due to the less amount of carbon black desorbed or fallen-off from the surface of each black magnetic composite particle, materials present in the system can be well dispersed together without any disturbance by the desorbed carbon black. Further, since irregularities are formed on the surface of each magnetic iron oxide particle by the carbon black adhered and bonded thereonto, the particles are prevented from contacting with each other, resulting in excellent dispersibility in a binder resin upon production of the magnetic toner.
The black magnetic toner according to the present invention obtained by using the above black magnetic composite particles adhered with a large amount of carbon black, can exhibit more excellent fluidity and blackness while maintaining as high a resistivity as not less than 1xc3x971013 xcexa9xc2x7cm.
The reason why the black magnetic toner of the present invention exhibits a more excellent fluidity, is considered as follows. That is, the black magnetic composite particles obtained by uniformly adhering a large amount of carbon black onto the surface of each magnetic iron oxide particle, are exposed to the surface of the black magnetic toner, thereby forming many fine irregularities thereon.
The reason why the black magnetic toner of the present invention exhibits a more excellent blackness, is considered by the present inventors as follows. That is, the black magnetic composite particles having a more excellent blackness are blended in the black magnetic toner.
The reason why the black magnetic toner of the present invention can maintain a high volume resistivity value nevertheless a large amount of carbon black is adhered onto the surfaces thereof, is considered by the present inventors as follow.
That is, in general, carbon black is present in the form of aggregated particles constituted from parallel-stacked crystallites each having a pseudo-graphite structure. Further, the carbon black fine particles are chemically and physically bonded with each other to form a cluster-like (grape-like cluster) structure. It is known that the larger the cluster-like structure, the higher the electrical conductivity of carbon black becomes. In the case where the carbon black fine particles having such a cluster-like structure are added to and mixed with a binder resin, those exposed to the surface of the magnetic toner also have the cluster-like structure, thereby increasing a conductivity of the magnetic toner. As a result, it is difficult to obtain a magnetic toner having a high volume resistivity value. On the contrary, in the case of the black magnetic composite particles according to the present invention, the carbon black coat is formed onto the surface of each magnetic iron oxide particle without forming the cluster-like structure. Therefore, since the magnetic toner using such black magnetic composite particles are also free from carbon black having the cluster-like structure, thereby maintaining a high volume resistivity value.
The black magnetic composite particles according to the present invention are not only more excellent in fluidity and blackness, but also show an excellent dispersibility in a binder resin due to a less amount of carbon black desorbed or fallen-off from the surface of each particle. Therefore, the black magnetic composite particles of the present invention are suitable as black magnetic particles capable of achieving a high image quality and a high copying speed.
Also, the black magnetic composite particles of the present invention have an excellent dispersibility, i.e., an excellent handling property and are, therefore, industrially advantageous.
The black magnetic toner obtained by using such black magnetic composite particles having more excellent fluidity and blackness, can also exhibit more excellent fluidity and blackness and is, therefore, suitable as black magnetic toner for achieving a high image quality and a high copying speed.
Further, the black magnetic toner of the present invention can maintain a high volume resistivity value nevertheless the use of black magnetic composite particles adhered with a large amount of carbon black. Therefore, the black magnetic toner of the present invention is suitable as high-resistant or insulating magnetic toner.